The present invention relates to fluid pumps and more particularly a variable displacement pump with electronic control for use in automobiles.
Power-assisted steering systems are systems used to aid drivers in controlling vehicles on the roads at low and high speeds by providing assistance to drivers in turning the steering wheel under various conditions. Power steering systems typically comprise a rack-and-pinion steering gear mechanism in which the gear rack is connected to a steering gear linkage. The rack engages a pinion gear that is connected to a driver operated steering shaft. A power steering pump is typically coupled to the rack-and-pinion gear mechanism to provide steering assistance to the rank-and-pinion gear as necessary. The power steering pump uses a rotary vane mechanism to control distribution of pressure from a power steering pump to the fluid motor portions of the rack-and-pinion steering gear mechanism. One type of power steering pump commonly used in automotive systems is the constant displacement rotary vane pump.
Constant displacement rotary vane pumps of the type used in power steering devices have a flow rate proportional to rotor speed. The steering gear supplied with pressurized hydraulic fluid from the pump requires high flow rates when vehicle speed is low and low flow rates when vehicle speed is high. With a constant displacement pump, however, the flow of hydraulic fluid from the pump is controlled by the rotor speed, and not by the amount of steering assistance needed. Excess hydraulic fluid is bypassed internally within the pump, creating heat and excess torque, which adversely affects fuel economy.
To improve the feel of a power steering system at all speeds and to make the system more fuel economical, conventional power steering systems may use electronic variable orifice (EVO) power steering systems. In an EVO power steering system, the fixed orifice of a power steering system is removed in the pump assembly and replaced with an EVO actuator. The EVO actuator is a flow control valve that is threaded onto the outlet of the pump which regulates flow rate as a function of vehicle speed as determined by an algorithm control. The EVO system works by providing high flow rates to the steering gear at low vehicle speeds (EVO actuator fully open) and lower flow rates as vehicle speeds increase (EVO actuator begins to close). A Hand Wheel Speed Sensor is typically used to in conjunction with the EVO system to increase steering assistance when it senses that the vehicle operator is making an evasive maneuver. The excess flow, which the pump produces in high or low speed situations, is normally bypassed internally within the pump.
Recently, improvements have focused on alleviating the excess flow that must be bypassed internally within the pump. To accomplish this, a variable displacement pump replaces the constant displacement pump. In conventional variable displacement pumps, a cam ring is provided movably in a pump casing. A pair of fluid pressure chambers serving as control chambers are formed in a gap between the cam ring and the pump casing, and the pressure on the upstream and downstream sides of an orifice provided midway in a discharge passage is made to act directly on the cam ring so as to move the cam ring against the urging force of a spring to change the volume of the pump chamber. Thus, in typical variable displacement pumps, output flow from the pump may be maintained at or near a constant rate regardless of engine speed.
Under ideal circumstances, the amount of pump flow needed varies with the amount of power assistance needed. For example, in situations where no power assistance is necessary, the pump flow through the pump would approach zero flow. When more assistance is necessary, the amount of pump flow correspondingly increases. Unfortunately, currently available variable displacement pumps do not approach zero flow when no power steering assistance is needed. The excess flow produced by the pump flows through the steering system and creates heat and excess torque, which adversely affect fuel economy.
It is thus an object of the present invention to provide a variable displacement pump having an electronically controlled variable relief valve that virtually eliminates excess flow by supplying only the amount of fluid required for steering assist at any speed and driving condition.
The above object is accomplished by providing a variable displacement pump having an electronically modulated solenoid relief valve for controlling fluid flow into the pump. An engine control module coupled to the electronic relief valve monitors vehicle speed and steering wheel turning rate change to adjust the pressure of fluid displacing the movable cam ring to provide for greater fluid flow when evasive maneuvers may be required and lesser fluid flow when low steering efforts are needed, thus resulting in energy and fuel savings.
The present invention has the added benefit that it adds little complexity to currently available pump housings.
Other objects and advantages of the present invention will become apparent upon considering the following detailed description and appended claims, and upon reference to the accompanying drawings.